EVE-NG images are pre-configured virtual machines that contain various network operating systems, such as Cisco IOS, Juniper JunOS, and Arista EOS. These images are used to create virtual network devices in the EVE-NG environment, allowing users to simulate real-world network scenarios. EVE-NG images are essentially QEMU (Quick EMUlator) images, which are compatible with the EVE-NG platform.